1. Field of the Invention
This invention relates to anhydrous monomer compositions crosslinkable by photoinitiators, and to a process for bonding non-transparent, particularly metallic, surfaces therewith.
2. Statement of Related Art
Adhesives based on radically polymerizable systems containing one or more monomers having ethylenically unsaturated double bonds are acquiring increasing significance. Systems such as these frequently contain short-chain, unsaturated carboxylic acid esters, such as methyl methacrylate, cyclohexyl methacrylate or tetrahydrofurfuryl methacrylate, free methacrylic acid, or mixtures of such monomers, methyl methacrylate being the most important component. Reactive adhesives such as these are mostly available both as so-called "two-component adhesives" and also an anaerobic systems.
One disadvantage of anaerobic systems is that they can only be hardened by radical polymerization in the absence of oxygen. In addition, acrylate adhesives formulated as two-component adhesives have limited pot lives.
It is also known that, provided suitable photoinitiators are used, vinyl monomers can be photopolymerized by electromagnetic radiation, cf. J Hutchinson et al., "Advances in Polymer Science" 14, 49 (1974), G. Oster and N. L. Yang, "Chemical Reviews" 68, 125 (1968). However, it has been found that relatively high-energy radiation is required to obtain bonds to good quality, i.e. having high tensile shear strengths, in acceptable setting times. If high-energy radiation cannot be used on technical grounds or if only weak radiation sources are available, long irradiation times have to be accepted. Finally, non-transparent substrates cannot be properly bonded with photoinitiated vinyl monomer systems under the effect of electromagnetic radiation under practical conditions.
U.S. Pat. No. 3,573,922 describes a process for the polymerization of solid, water-soluble vinyl monomers, namely salts and water-soluble amides, in which a catalyst mixture of sensitizer and an organic sulfine, phosphine or arsine compound is added to the monomer mixture and the system is subjected to photopolymerization. According to the specification, it is thus possible to produce layers which may be used as printing plates or the like by polymerization of the exposed areas so that they are resistant to the action of solvents or the like, while unexposed areas are washed out by suitable solvents, so that plates corresponding to the light pattern can be produced. The water-soluble vinyl monomers disclosed in U.S. 3,573,922 are not suitable as a basis for reactive adhesives.